The wattage on the A-S301 is a little lower than the R-S500 but the build quality is a little more robust in the amp section of the A-S301. (dual heat sinks and slightly larger capacitors) According to Yamaha the circuit design is little more refined in the integrated amps in comparison to the receiver line.
The A-S301 offers digital inputs as well. That feature alone makes it worth it IMO.